Atmosphere Threatened By Nitrogen Pollutants Entering Ocean

May 16, 2008 — A large quantity of nitrogen
compounds -- emitted into the atmosphere by humans through the burning
of fossil fuels and the use of nitrogen fertilizers -- enters the
oceans and may lead to the removal of some carbon dioxide from the
atmosphere, concluded a team of international scientists led by Texas
A&M University Distinguished Professor of Oceanography and
Atmospheric Sciences Robert Duce.

The team of 30 experts from institutions around the world presented its conclusions in the current issue of the journal Science.

Human-caused atmospheric nitrogen compounds are carried by wind and
deposited into the ocean, where they act as a fertilizer and lead to
increased production of marine plant life. The increase in plant life
causes more carbon dioxide to be drawn from the atmosphere into the
ocean. This process results in the removal of about 10 percent of the
human-caused carbon dioxide in the atmosphere, thus potentially
reducing the climate warming potential, according to the team's paper.

However, some of the nitrogen deposited in the ocean is re-processed
to form another nitrogen compound called nitrous oxide, which is then
released back into the atmosphere from the ocean. Nitrous oxide is a
powerful greenhouse gas itself -- about 300 times more powerful per
molecule than carbon dioxide -- thus cancelling out about two-thirds of
the apparent gain from the carbon dioxide removal, Duce explained. "But
of course, the whole system is so complex that we're still rather
unsure about what some of the other impacts might be within the ocean,"
he said.

In most areas of the ocean, nitrogen is the nutrient that limits the
production of plant life, Duce said. So when all of the nitrogen in an
area of the surface ocean is used up, no more plant life forms in that
area. The team found that human-caused nitrogen deposits account for up
to one-third of the external input of nitrogen into the ocean, and this
increase in nitrogen available for the production of plant life causes
more plants to form, Duce explained.

Oceanic plant life is produced from marine carbon (bicarbonate) in
the ocean, and that amount of bicarbonate is in equilibrium with the
carbon dioxide in the atmosphere. So when more bicarbonate is used up
to produce marine plant life, it disrupts the equilibrium, and carbon
dioxide is drawn down to the ocean from the atmosphere to restore the
balance, Duce explained.

Thus, the human-caused nitrogen fertilization of the ocean removes
some of the most important greenhouse gas -- carbon dioxide -- from the
atmosphere, Duce said. This gain, however, is offset by the nitrogen
compound, nitrous oxide, that also forms in the ocean due to the
nitrogen fertilization and is re-emitted into the atmosphere as a
powerful greenhouse gas, he added.

"If you don't consider the impact of human-caused nitrogen when
trying to model climate change, you're missing a possibly significant
part of the overall carbon cycle as well as the nitrogen cycle," Duce
said. "So nitrogen deposition is potentially a very important factor in
the climate change issue."

According to the team's calculations, about 54 million tons of
nitrogen produced from human activities entered the ocean from the
atmosphere in the year 2000. The team also found that the current
nitrogen emissions are about 10 times what they were in 1860, Duce
said. He added that the amount of nitrogen entering the atmosphere is
expected to rise in the coming decades with the increase in demand for
energy and fertilizers, and the team estimates that by the year 2030,
human-caused nitrogen emissions will have risen to around 62 million
tons per year.

"Clearly, there is much that we do not know about the extent and
timescale of the impacts of this nitrogen deposition on the oceans and
the subsequent feedbacks to the climate system," Duce said. "The
implications are complex and interactive, and this is a very important
issue that policy makers need to address and that scientists trying to
model and understand the future of climate and climate change need to
take into consideration."